(bug 50303) improve cache key for shared cache.

This allows any object cache keys shared between clients
(and the repo) to be based on by a) a version ID
and b) the repo's name.

TODO: make the default value use the repo ID per default.

DEPLOYMENT NOTE:

This causes cached objects to NOT be shared between wikis per
default (see TODO)! When deploying on a wiki farm, make sure
to set the sharedCacheKeyPrefix option appropriately:
It should be based on the desired repo's database name and
a version ID indicating the version of the deployed code.
Suggested pattern: <repo>:deploy/<version>

Change-Id: I78009d3eb1aed2b42ee53acf6f7bc812e7ee836f
Merge "(bug 50303) improve cache key for shared cache."